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UNIVERSITÀ DI GENOVA
FONDAZIONE ISTITUTO ITALIANO DI TECNOLOGIA (IIT)
Doctoral School on “Nanochemistry”
XXX Cycle
Production and processing of graphene and 2D
crystal-based inks for energy conversion devices
Doctor of Philosophy (Ph.D.) Thesis
Ph.D. candidate:
Leyla Najafi
Supervisor:
Dr. Francesco Bonaccorso
(Graphene Labs, Istituto Italiano di Tecnologia)
CO-Supervisors:
Dr. Vittorio Pellegrini
(Graphene Labs, Istituto Italiano di Tecnologia)
Prof. Daniele Marrè
(Department of Physics, University of Genova)
Tutor:
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Reviewers:
Prof. Aldo Di Carlo
( Dipartimento di Ingegneria Elettronica, Università di Roma)
Prof. Xinliang Feng
(Technische Universitaet Dresden)
Thesis Submission: February 2018
Thesis Dissertation:
March 2018
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Mom and Dad, I have no words to acknowledge the
sacrifices you made and the dreams you had to let go,
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Acknowledgement
I am very happy and thankful to achieve at this stage in my life and career. Surely, I could not get where I am without the help and support of many people.
Firstly, I am grateful to my supervisor, Dr. Francesco Bonaccorso , His expertise, understanding, patience, generous guide and support, made it possible for me to work on a topic that was a great interest to me. It was pleasure working with him.
I am grateful to Dr. Vittorio Pellegrini for giving me full access to the research facilities at Graphene Labs “Istituto Italiano di Tecnologia” that allowed me for carrying out this research work.
I would like to express my gratitude to Prof. Daniele Marrè, my academic Co-supervisor at Department of Physics “ University of Genova”.
I would like to express my sincere gratitude to my mentor and friend Dr. Sebastiano Bellani for the continuous support of my Ph.D. thesis and related research, for his patience, motivation, and immense knowledge. His guidance helped me in all the time of research and writing of this thesis. I could not have imagined having a better advisor and mentor for my Ph.D. study.
I would like to acknowledge the valuable input of Dr. Antonio Esau Del Rio Castillo, who contributed to many stimulating discussions that helped to shape this projects.
My sincere thanks also go to Dr. Aldo Di Carlo and his group at Dept. Electronics Engineering, University of Rome, who provided me an opportunity to join their team, and who gave access to the laboratory and research facilities. Without their precious support it would not be possible to conduct this research.
My thanks also go out to the technicians of graphene lab, Elisa Mantero, Luca Gagliani, Manuel Crugliano. This work would be not materialized without their support. It was a great pleasure and honor working with them.
I would like to express my gratitude and many thanks to all my friends that made my time in IIT
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My Special thanks go to my husband Dr. Reinier Oropesa-Nuñez, without his patience and support I couldn’t make it. He made my road easier to walk on it and gave me the strength to continue over adversities.
Last but not the least, I would like to thank my family, specially my Mom and Dad for supporting me spiritually and unconditional throughout writing this thesis and my life in general.
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Scientific Publications
1. Engineered MoSe2-based heterostructures for efficient electrochemical hydrogen evolution reaction
L. Najafi, S. Bellani, R. Oropesa-Nuñez, A. Ansaldo, A. E. Del Rio Castillo, M. Prato, and F.
Bonaccorso. Adv. Energy Mater. 1703212, (2018).
2. Doped-MoSe2 nanoflakes/3d metal oxide-hydr(oxy)oxides hybrid catalysts for pH-universal electrochemical hydrogen evolution reaction
L.Najafi, S. Bellani, R. Oropesa-Nuñez, A. Ansaldo, M. Prato, A. E. Del Rio Castillo and F.
Bonaccorso. (submitted)(2018).
3. Conductive ITO nanoparticles break optical transparency/high-areal capacitance trade-off for advanced aqueous supercapacitors.
S. Bellani, L. Najafi, G. Tullii, A. Ansaldo, R. Oropesa-Nuñez, M. Prato, M.Colombo, M. R. Antognazza, F. Bonaccorso. J. Mater. Chem. A. 5, 25177-25186 (2017).
4. Extending the continuous operating lifetime of perovskite solar cells with a molybdenum disulfide hole extraction interlayer
G. Kakavelakis, I. Paradisanos, B. Paci, A. Generosi, M. Papachatzakis, T. Maksudov, A. E. Del Rio Castillo, L. Najafi, G. Kioseoglou, E. Stratakis, F. Bonaccorso, and E. Kymakis. Adv. Energy Mater., 1702287, (2018).
5. Thioethyl-porphyrazine/Nanocarbon Hybrids for Photoinduced Electron Transfer.
S. Belviso, A. Capasso, E. Santoro, L. Najafi, F. Lelj, S. Superchi, D. Casarini, C. Villani, D. Spirito, S.Bellani, A. E. Del Rio Castillo, and F. Bonaccorso. (submitted)(2018).
6. Carbon coated MoS2 flakes as anode for lithium-ion batteries.
D. A. Dinh, H. Sun, L. Najafi, A. E. Del Rio Castillo, A. Ansaldo, Z. Dang, C. Di Giovanni, V. Pellegrini, and F. Bonaccorso. (submitted)(2018).
7. Graphene and 2D materials for high efficient and stable perovskite solar cells.
A. Agresti, S. Pescetelli, L. Najafi, F. Bonaccorso, Y. Busby and A. Di Carlo, IEEE NANO 2017.
8. Solution-Processed Hybrid Graphene Flake/2H-MoS2 Quantum Dot Heterostructures for Efficient Electrochemical Hydrogen Evolution.
L. Najafi, S. Bellani, B. Martin-Garcia, R. Oropesa-Nuñez, A. E. Del Rio Castillo, M. Prato, I.
Moreels, and F. Bonaccorso, Chem. Mater.29, 5782–5786 (2017).
9. Graphene-Based Hole-Selective Layers for High-Efficiency, Solution-Processed, Large-Area, Flexible, Hydrogen-Evolving Organic Photocathodes.
S. Bellani, L. Najafi, B. Martin-Garcia, A. Ansaldo, A. E. Del Rio Castillo, M. Prato, I. Moreels
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10. Few-layer MoS2 flakes as a hole-selective layer for solution-processed hybrid organic hydrogen evolving Photocathodes.
S. Bellani, L. Najafi, A. Capasso, A.E. Del Rio Castillo, M. Rosa Antognazza and F. Bonaccorso, J. Mater. Chem. A. 5, 4384-4396 (2017).
11. Ruthenium Tetrazole Based Electroluminescent Device: Key Role of Counter Ions for Light Emission Properties.
H. Shahroosvand, L. Najafi, A. Sousaraei, E. Mohajerani, M. Janghouri, and F. Bonaccorso, J.
Phys. Chem. C.120, 24965–24972 (2016).
12. Few-Layer MoS2 Flakes as Active Buffer Layer for Stable Perovskite Solar Cells.
A. Capasso, F. Matteocci, L. Najafi, M. Prato, J. Buha, L. Cinà, V. Pellegrini, A.Di Carlo, F.
Bonaccorso, Adv. Energy Mater.6, 1600920 (2016).
13. Spray deposition of exfoliated MoS2 flakes as hole transport layer in perovskite-based photovoltaics.
A. Capasso, A.E. Del Rio Castillo, L. Najafi , V. Pellegrini, F. Bonaccorso, F. Matteocci, L. Cinà, A. Di Carlo. IEEE NANO 2015.
Communications at Conferences
Orals1. S. Bellani, F. Bonaccorso, L. Najafi, M. Prato, A. Del Rio, A. Ansaldo, I. Moreels, B. Garcia, R. Oropesa-Nuñez “Graphene Related 2D Crystals and Hybrid Systems for High-Efficiency, Solution-Processed, Large-Area, Flexible, Stable Electrocatalysts and Photocathodes for Hydrogen Evolution Reaction” MRS 2017, 26-1 Dec. Boston.
2. D. A. Dinh, H. Sun, L. Najafi, A. E. Del Rio Castillo, A. Ansaldo, C. Di Giovanni, V.
Pellegrini and F. Bonaccorso “Facile synthesis of MoS2-flakes/amorphous-carbon
composite as anode for lithium-ion Batteries” Applied Nanotechnology and
Nanoscience International Conference 2017, 18-20 Oct. 2017, Rome, Italy.
3. D. A. Dinh, H. Sun, L. Najafi, C. Di Giovanni, A. E. Del Rio Castillo, A. Ansaldo, V.
Pellegrini, and F. Bonaccorso, “Carbon coated MoS2 flakes as anode for lithium-ion
batteries“ NanoMaterials for Energy and Environment, 28 - 30 June 2017, Paris, France.
4. S. Bellani, L. Najafi, B. Martín-García, A. Ansaldo, A. E. Del Rio Castillo, M. Prato, I. Moreels and F. Bonaccorso “Graphene-based Hole Selective Layers for High-efficiency,
Solution-processed, Large-area, Flexible, Stable Hydrogen-Evolving Organic
Photocathodes” Graphene 2017, 28-31 March 2017, Barcelona, Spain.
5. L. Najafi, S. Bellani, A. Capasso, A. E. Del Rio Castillo, M. R. Antognazza, and F.
Bonaccorso, “Few-layer MoS2 Flakes as Hole-selective Layer for Solution-processed
Hybrid Organic Hydrogen-evolving Photocathodes” Graphene 2017, 28-31 March 2017,
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6. A. Capasso, F. Matteocci, L. Najafi, V. Pellegrini, A. Di Carlo, F. Bonaccorso, “Stable
perovskite solar cells with a MoS2 active buffer layer” RPGR 2016, 25-29 September
2016, Seoul, Korea.
7. A. Capasso, F. Matteocci, L. Najafi, M. Prato, V. Pellegrini, A. Di Carlo, F. Bonaccorso,
“MoS2 flakes as hole transport layer in perovskite-based photovoltaics” Graphene Week
2016, June 13-17, 2016, Warsaw, Poland.
8. A. Capasso, F. Matteocci, L. Cinà, A.E. Del Rio Castillo, L. Najafi, V. Pellegrini, A. Di Carlo,
F. Bonaccorso “Spray deposition of exfoliated MoS2 flakes as hole transport layer in
perovskite-based photovoltaics” IEEE Nano 2015, 27-30 July, Rome (Italy).
Posters
1. L. Najafi, A. Capasso, F. Matteocci, M. Prato, J. Buha, L. Cinà,V. Pellegrini, A. Di Carlo, F.
Bonaccorso “Few-layer MoS2 flakes as active buffer layer for stable perovskite solar
cells.” 2st International Conference on Perovskite Solar Cells and Optoelectronic. 26-28
September 2016, Genova, Italy.
2. L. Najafi , A. Capasso, A.E. Del Rio Castillo, V. Pellegrini, F. Bonaccorso, F. Matteocci, L.
Cinà, A. Di Carlo. “Stable perovskite solar cells with spray deposition of exfoliated MoS2
flakes as hole transport”.XXth International Krutyn Summer School. 12-18 June 2016,
Warsaw, Poland.
3. L. Najafi, A. Capasso, F. Matteocci, M. Prato, J. Buha, L. Cinà,V. Pellegrini, A. Di Carlo, F.
Bonaccorso.” Exfoliated MoS2 flakes as hole transport layer in perovskite-based
photovoltaics.” Graphene 2016. 19-22 April 2016, Genova, Italy. (Winner of student poster award)
4. S. Belviso, A. Capasso, E. Santoro, L. Najafi, F. Lelj, S. Superchi, D. Casarini, C. Villani, D. Spirito, S.Bellani, A. E. Del Rio Castillo, and F. Bonaccorso. “Supramolecular hybrids of thio-ethylporphyrazine with graphene and carbon nanotubes for photo induced electron transfer.” Graphene 2016 . 19-22 April 2016 , Genova, Italy.
5. L. Najafi, A. Capasso, F. Matteocci, M. Prato, J. Buha, L. Cinà,V. Pellegrini, A. Di Carlo, F.
Bonaccorso. “MoS2 flakes as a hole transport material for stable perovskite solar cells. “
1st International Conference on Perovskite Solar Cells and Optoelectronic. 27-29 September 2015, Lausanne, Switzerland.
6. L. Najafi, S. Belviso, A. Capasso, E. Santoro, F. Lelj, S. Superchi, D. Casarini, C. Villani, D. Spirito, S.Bellani, A. E. Del Rio Castillo, and F. Bonaccorso. “Hybrids of thio-ethylporphyrazine with nano carbons for photo induced electron transfer.” International Conference on Hybrid and Organic Photovoltaics. 10-13 May 2015, Rome, Italy.
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Table of Contents
LIST OF ACRONYMS AND ABBREVIATIONS 16
INTRODUCTION 20
CHAPTER 1: Two-Dimensional Material (2D) Families and Their Properties 26
1.1 Graphene 26 1.1.1 Electronic Properties 29 1.1.2 Optical Properties 30 1.1.3 Mechanical Properties 31 1.1.4 Thermal Properties 32 1.2 Other 2D Materials 32
1.3 Production Techniques of 2D Materials 34
1.3.1 Bottom-up Approach 35
1.3.1.1 Growth of Graphene on Silicon Carbide (SiC) 35
1.3.1.2 Chemical Vapor Deposition (CVD) 36
1.3.2 Top-Down Approach 39
1.3.2.1 Mechanical Cleavage (MC) 39
1.3.2.2 Anodic bonding 39
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1.4 Applications of 2D Materials 42
CHAPTER 2: Experimental Methods 45
2.1 Materials Production 45
2.1.1 Preparation of Single-/Few-layer 2H-MoS2, 2H-MoSe2 and Graphene Flakes
Dispersions by LPE 45
2.1.2 Chemical Exfoliation of Single-/Few-Layer 1T-MoS2 Flakes Dispersion by
Li-Intercalation Method 46
2.1.3 One-step Synthesis of 2H-MoS2 Quantum Dots (QDs) by Solvothermal
Method 46
2.1.4 Preparation of Graphene Oxide (GO)by Modified Hummer’s Method 47
2.1.5 Preparation of Reduced Graphene Oxide (RGO)by Modified Hydrothermal
Method 47
2.1.6 Debundling and Dissolution of Single-walled Carbon Nanotubes (SWNTs) 48
2.1.7 Functionalization of 2D Materials Films 48
2.1.7.1 HAuCl4-Chemical Doping-of MoS2 Flakes Films 48
2.1.7.2 Thermal Texturization of MoSe2 Flakes Films 48
2.1.7.3 Chemical Treatment-induced Phase Conversion of MoSe2 Flakes Films 49
2.1.7.4 Transition Metal Chloride-Chemical Doping of MoSe2 Flakes 49
2.1.7.5 Post-synthesis Silane Functionalization of GO and RGO 50
2.2 Characterization Techniques and Instrumentation 50
CHAPTER 3: Characterization of 2D Material Dispersions 53
3.1 Graphene Flakes 53
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3.1.2 Raman Spectroscopy Analysis 53
3.1.3 Morphological Characterization 55
3.1.4 X-ray Photoelectron Spectroscopy Analysis 56
3.2 Graphene Oxide and RGO Flakes 57
3.2.1 Optical Absorption Spectroscopy Analysis 57
3.2.2 Raman Spectroscopy Analysis 58
3.2.3 Morphological characterization of GO and RGO flakes 60
3.2.4 X-ray Photoelectron Spectroscopy Analysis 61
3.2.5 Ultraviolet Photoelectron Spectroscopy Analysis 62
3.3 Functionalized Graphene Oxide (fGO) and Functionalized Reduce Graphene
Oxide (f-RGO) Flakes 63
3.3.1 Morphological characterization 63
3.3.2 X-ray Photoelectron Spectroscopy Analysis 65
3.3.3 Ultraviolet Photoelectron Spectroscopy Analysis 66
3.4 2H-MoS2 Flakes, 1T-MoS2 Flakes and 2H-MoS2 QDs 67
3.4.1 Optical Absorption Spectroscopy Analysis 68
3.4.2 Photoluminescence Characterization 68
3.4.3 Raman Spectroscopy Analysis 69
3.4.4 Morphological Characterization 71
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3.4.6 X-ray Photoelectron Spectroscopy Analysis 73
3.5 2H-MoSe2 Flakes 75
3.5.1 Optical Absorption Spectroscopy Analysis 75
3.5.2 Raman Spectroscopy Analysis 76
3.5.3 Morphological Characterization 78
3.5.4 X-ray Diffraction Measurements 79
3.5.5 X-ray Photoelectron Spectroscopy Analysis 80
3.6 Single-wall Carbon Nanotubes 81
3.6.1 Optical Absorption Spectroscopy Analysis 81
3.6.2 Raman Spectroscopy Analysis 82
3.6.3 Morphological Characterization 83
CHAPTER 4: Transition Metal Dichalcogenides (TMDs) for Electrochemical
Hydrogen Evolution Reaction (HER) 85
4.1 Fundamentals of HER 85
4.1.1 Over potential, Tafel Slope, Exchange Current Density and Faradaic
Efficiency 85
4.1.2 Electrochemical Measurements 87
4.2 Electrodes Fabrication and Electrochemical Characterization 88
4.2.1 Hybrid Graphene Flake/2H-MoS2 QDs Heterostructures for Efficient
Electrochemical HER 88
4.2.1.1 Fabrication of the Electrodes 90
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4.2.1.3 Electrochemical Characterization 91
4.2.2 Engineered MoSe2-based Heterostructures for Efficient Electrochemical
HER 93
4.2.2.1 Fabrication of the Electrodes 95
4.2.2.2 Electrodes Characterization 95
4.2.2.2.1Graphene/MoSe2 and SWCTNs/MoSe2 heterostrcutures 95
4.2.2.2.2Engineering of the Electrode 100
4.2.2.3 Electrochemical Characterization 104
4.2.3 Non-Noble Metal Chloride Charge-Transfer Doping of MoSe2 Flakes for
Efficient PH-Universal Electrochemical HER 110
4.2.3.1 Fabrication of the Electrodes 111
4.2.3.2 Electrodes Characterization 112
4.2.3.3 Electrochemical Characterization 119
4.3 Summary 123
CHAPTER 5: Solar Water Splitting 125
5.1 Photoelectrochemical (PEC) Cells 125
5.2 Solar-to-hydrogen Conversion Efficiency (ƞSTH) 126
5.3 Hybrid Organic H2-evolving Photocathode 127
5.4 Photoelectrochemical Measurements 128
5.5 Two Dimensional (2D) Material Interfaces Engineering 129
5.5.1 MoS2 Flakes as a HSL for Solution-Processed Hybrid Organic H2 Evolving
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5.5.1.1 Architecture of the MoS2-based Organic Photocathodes 130
5.5.1.2 Working Principles of the MoS2-based Organic Photocathodes 131
5.5.1.3 Characterization of the MoS2-based Organic Photocathodes 132
5.5.1.4 Photoelectrochemical Characterization 135
5.5.2 Graphene-Based HSL for High-Efficiency and Flexible, H2-Evolving Organic
Photocathodes 139
5.5.2.1 Architecture of Graphene Derivative-based Organic Photocathodes 140
5.5.2.2 Working Graphene Derivative-based Organic Photocathodes 141
5.5.2.3 Characterization of Graphene Derivative-based Organic Photocathode 142
5.5.2.4 Photoelectrochemical Characterization 147
5.5.2.5 Flexible and Large Area Photocathodes 154
5.6 Summary 155
CHAPTER 6: Two Dimensional (2D) Material Interfaces Engineering Perovskite
Solar Cells (PSCs) 157
6.1 Perovskite Solar Cells (PSCs) 157
6.2 Device Fabrication 159
6.2.1 One Step Process Fabricating Perovskite Layer 159
6.2.2 Two Step Process Fabricating Perovskite Layer 159
6.3 Solar Cell Characterization 160
6.4 Solar Cell Measurements 162
6.5 High Performance and Stable Perovskite Hybrid Solar Cells based on
Few-Layer MoS2 Flakes
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6.5.1 Few-Layer MoS2 Flakes as Active Buffer Layer for Stable PSCs 163
6.5.1.1 Architecture of PSCs 164
6.5.1.2 Morphological Characterization of the PSCs 165
6.5.1.3 Photovoltaic Performance 166
6.5.2 Ambient Stable and Scalable Perovskite Hybrid Solar Cells Using MoS2 as
Hole Transport Interlayer 172
6.5.2.1 Architecture of PSCs 173
6.5.2.2 Photovoltaic Performance 173
6.6 Summary 185
CHAPTER 7: Conclusion and Future Directions 187
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List of Acronyms and Abbreviations
µ Mobility
0D Zero-dimensional
1D One-dimensional
2D Two-dimensional
3D Three-dimensional
ABL Active buffer layer
AFM Atomic force microscopy
AgCl Silver chloride
b Tafel Slope BHJ Bulk heterojunction BN Boron nitride CB Conduction band Cd2+ Cadmium cations CNTs Carbon nanotubes Co2+ Cobalt cations CSL Charge-selective layer Cu2+ Copper cations
CVD Chemical vapor deposition
DI Deionized water
DMF N,N-dimethylformamide
DMSO Dimethyl sulfide
DSSC Dye-sensitized solar cell
E applied Applied potentials
EC Electrocatalyst
EDX Energy-dispersive X-ray spectroscopy
EIS Electrochemical impedance spectroscopy
EQE External quantum efficiency
ESL Electron-selective layer
ETL Electron transporting layer
F4TCNQ 2,3,5,6-Tetrafluoro-7,7,8,8-tetracyanoquinodimethane
FE Faradaic Efficiency
Fe2+ Iron cations
FF Fill factor
FGO Functionalized graphite oxide
FLG Few layers graphene
FoM Figures of Merit
F-rGO Functionalized reduced graphene oxide
FTO Fluorine-doped tin oxide
FWHM Full width half maximum
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GO Graphene oxide
H2O Water
H2SO4 Sulfuric acid
H3O+ Hydronium ion
Hads Hydrogen adsorbed
HAuCl4 Chloroauric acid
HER Hydrogen evolution reaction
HOMO Highest occupied molecular orbital
HOPG Highly ordered pyrolytic graphite
HSL Hole selective layers
HTL Hole transporting layer
i0 Exchange current
IPA 2-Propanol
IPCE Incident photonto current efficiency
IR Infrared
ITO Indium tin oxide
j0 Exchange currenty density
J0V vs RHE Photocurrent at 0 V vs. RHE
Jsc Short circuit current
J-V curve Current-voltage measurements
K points Dirac points
KCl Potassium chloride
KMnO4 Potassium permanganate
LED Light-emitting diode
Li+ Lithium cations
LIB Lithium-ion battery
LiOH Lithium hydroxide
Li-TFSI Lithium bis(trifluoromethanesulfonyl)imide
LPE Liquid phase exfoliation
LSV Linear sweep voltammetry
LUMO Lowest unoccupied molecular orbital
m* Mass
MAI Methylammonium iodide
MAPbBr3 Methyl-ammonium lead bromide
MAPbI3 Methyl-ammonium lead iodide
MC Mechanical cleavage
MCl2 Transition metal chloride
MoO3 Molybdenum trioxide
MoS2 Molybdenum diselenide
MoSe2 Molybdenum disulfide
MPTMS 3-mercaptopropyl)trimethoxysilane
MWCNT Multi walled carbon nanotubes
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NaNO3 Sodium nitrate
n-BuLi n-butyl lithium
Ni2+ Nickel cations
NMP N-methyl-2-pyrrolidone
Ƞsth Solar-to-hydrogen conversion efficiency
OAS Optical absorption
OC Open circuit
OER Oxygen evolution reaction
PbBr2 Lead(II) bromide
PbCl2 Lead(II) chloride
PbI2 Lead(II) iodide
PCBM Phenyl-C61-butyric acid methyl ester
PCE Energy conversion efficiency
PEC Perovskite solar cell
PEDOT:PSS Poly(3,4-ethylenedioxythiophene) polystyrene sulfonate
PEM Proton exchange membrane
PET Polyethylene terephthalate
PL Photoluminescence
Pmax Maximum power point
Pos Position
PSC Photo-electrochemical cell
Pt Platinum
PTAA Poly(triaryl amine)
PV Photovoltaic
q Charge of the carrier
QDs Quantum Dots
QDSC Quantum dot solar cells
Ra Roughness
RBMs Radial breathing modes
RGO Reduced graphene oxide
RHE Reversible hydrogen electrode
rr-P3HT Poly(3-hexylthiophene-2,5-diyl)
SBS Sedimentation-based separation
SC Short circuit
SEM Scanning electron microscopy
SIB Sodium-ion battery
SiC Growth on silicon carbide
SLG Single layers graphene
Spiro-OMeTAD 2,2’,7,7’-tetrakis(N,N-di-p-methoxyphenylamine)-9,9’-spirobifluorene
SWCNT Single walled carbon nanotube
T Transmittance
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TCO Transparent conductive oxide
TEM Transmission electron microscopy
TiO2 Titanium dioxide
TMD Transition metal dichalcogenides
TPV Transient photovoltage measurements
UPS Ultraviolet photoelectron spectroscopy
UV Ultraviolet
VB Valence band
VBM Valence band maximum
Vmpp Potential at maximum power point
Vo Onset potential
Voc Open circuit voltage
WF Work function
XPS X-ray photoelectron spectroscopy
XRD X-ray diffraction
Zn2+ Zinc cations
ZnO Zinc oxide
α Absorption coefficient
Γ Brillouin zone center
γ Surface energy
ΔGH0 Gibbs free energy
ηF Current-to-hydrogen faradaic efficiency
λ Wavelength
π Bonding molecular orbital
π* Antibonding molecular orbital
τ Average scattering time
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Introduction
In recent years the focus on renewable energy sources has been raised due to increasing oil prices and the growing concern for global warming. Currently, most of the world’s energy consumption is fossil fuel based, but there is an increasing interest in replacing the fossil fuels with renewable resources. In this context, solar energy represents the most significant contribution of energy worldwide, which, although has low power density, could potentially satisfies the global energy demand on its own. However, several challenges must be overcome to make solar energy viable and competitive on a large scale. For example, enhancing the performance of solar energy conversion systems through increased efficiency and use of durable materials; reducing the cost of the material, fabrication and installation, so that these systems can be deployed on a large scale.
Hydrogen (H2), as energy carrier, is one of the most promising long-term solutions for renewable
energy, due to its low environmental impact and high energy density (between 120-140 MJ kg-1).
However, H2 is mostly produced from fossil fuels, which are limited in supply and create harmful
CO2 emissions. Photo-electrochemical (PEC) water splitting, a process in which H2O is split into H2
and O2 using the energy from sunlight, is a promising technology for renewable hydrogen
production. Efficient, inexpensive and electrochemically stable materials must be developed to make viable and widespread PEC water splitting devices implementation. However, essential barriers such as the creation of active catalysts, corrosion prevention strategies, and techniques for successfully integrating all required components of the PEC device must be overcome. Essential barriers also remain standing in the pathway of photovoltaic energy conversion efficiency. The continuous development of novel device concepts, materials, and fabrication processes has overcome these obstacles partially, contributing to decreasing the cost of solar power. Recently, perovskite solar cells (PSCs), considered as a promising direction for low-cost and highly efficient energy conversion, have shown a rapid growth of efficiency from 3.8% to 22.7 %. PCSs absorbers possess several distinctive features including broad light absorption from the visible to the near-infrared region, a high extinction coefficient, large charge carrier diffusion lengths, tunable optical properties and low-temperature solution processability. These aforementioned features are attracting considerable attention in the photovoltaic industry. However, further research in PSCs regarding the enhancement of their performance and stability,
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lowering their fabrication costs, as well as developing environmentally benign perovskites is required.
In this scenario, graphene, a one-atom-thick planar sheet of carbon atoms densely packed in a honeycomb crystal lattice, has attracted great interest in recent years due to its high specific
surface area and the excellent mechanical, electrical, optical and thermal properties. The
advantage of these unique features has been exploited using graphene as a component of advanced (opto)electronic devices (e.g., high-frequency devices, touch screens, flexible and wearable devices, ultrasensitive sensors/photodetectors, light emitting diodes and ultrafast lasers), as well as novel energy storage and conversion systems, including batteries, supercapacitors, as well as solar and photo-electrochemical (PEC) cells. However, graphene is a material without an electronic bandgap, making it deemed unfavorable as active component in application requiring semi-conductive properties. Therefore, other 2D semiconducting materials have been sought-after. In particular, transition metal dichalcogenides (TMDs), group III and IV compounds, and graphene analogues such as boron nitride (BN), typically exhibit strong in-plane covalent bonding and weak out-of-plane van der Waals interactions through the interlayer gap. These features, together with the quantum confinement and surface effects, are the reason for many interesting layering-dependent properties found in atomically thin 2D materials nanosheets but no on their bulk counterparts. For example, some bulk materials are semiconductors with indirect band gaps, while their single-layer nanosheets are semiconductors with direct band gaps, resulting in dramatic changes of their properties such as the enhancement of photoluminescence. The physical and chemical properties of 2D materials can also be related to the interlayer distance which triggers a series of regulations in the band gap, conductivity, thermoelectric and photovoltaic properties and superconductivity. In particular, varying the interlayer distance of 2D materials, it is possible to obtain novel heterostructures properties, which may not be achieved in the initial materials. A larger interlayer spacing also means further active sites, an ion-accessible surface in the interlayer space, accessible for catalysis. The latter will considerably enhance the performance of 2D materials in energy storage devices (e.g., lithium-ion battery (LIB), sodium-ion battery (SIB) and supercapacitor), and energy conversion devices (e.g., solar cells, fuel cells). My Ph.D. research aimed to design and synthesize 2D materials for the production of high performance 2D materials-based (opto)electronic devices. Firstly, I produced graphene and 2D-TMDs from their parent bulk crystals in suitable liquids to yield dispersions by liquid phase
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exfoliation (LPE) or chemical methods (Li-intercalation).This allowed the formulation of functional
inks, which can be processed by large-scale, cost-effective solution processed techniques reaching
high-electrocatalytic performance (ƞ10 of 100 mV and cathodic current density > 100 mA cm-2 at ƞ
inferior to 200 mV) compatible with high-throughput industrial implementation focused on developing high-volume liquid-phase and chemical exfoliation for a wide variety of layered materials. These techniques have been optimized to control the flake size and to increase the edge-to-surface ratio, which is crucial for optimizing electrode performance in the final applications.
At the next step, I widened my activity towards the implementation of novel 2D materials-based (photo)electrochemical cell and solar cell platforms. In fact, the development of novel, sustainable
methods for scalable and efficient hydrogen (H2) production, as well the increase of the efficiency
and the stability of the PSCs represented the main challenges of my activity. In order to attempt
this, I exploited 2D materials as electro catalysts for H2 evolution reaction and hole selective layers
(HSLs) in organic H2-evolving photocathodes. In detail, I designed solution-processed hybrid
heterostructures based on carbon nanomaterials and 2D-TMDs, Which showing high PEC activity in different pH conditions, i.e., ranging from acid to basic. Afterward, by tuning the electrochemical properties of 2D materials, I exploited graphene derivatives and 2D-TMDs as HSL for boosting the efficiency and the durability of PEC. The 2D material-based interface engineering avoids the recombination loss by preventing recombination defects between the different interface layers. Using 2D materials as interlayer permitted to achieve record high performances
concerning all-solution-processed photocathodes (i.e., photocurrent at 0 V vs. RHE (J0V vs RHE) of
-6.01 mA cm-2, onset potential (Vo) of 0.6 V vs. RHE, ratiometric power-saved efficiency (φsaved) of
1.11% and operational activity of 20 hours). Moreover, the photocathodes are demonstrated to be
effective in different pH environment ranging from acid to basic, showing J0V vs RHE exceeding 1 mA
cm-2. This is pivotal for their exploitation in tandem configurations, where photoanodes operate
only in restricted electrochemical conditions. Lastly, I demonstrated the up-scaling feasibility of
the as-produced devices by fabricating a large-area (9 cm2) flexible (onto ITO-PET substrate)
photocathodes, with remarkable φsaved of 0.31%.
In parallel, I exploited 2D material-based interface engineering also for improving the photovoltaic
performance of the PSC. In particular, I successfully introduced 2D MoS2 film in between the
2,2’,7,7’-tetrakis(N,N-di-p-methoxyphenylamine)-9,9’-24
spirobifluorene (spiro-OMeTAD), and the perovskite absorber, i.e., CH3NH3PbI3 (known as MAPbI3),
to enhance the efficiency of PSCs. Besides the high power conversion efficiency value achieved
(>20%), the addition of 2D-MoS2 film significantly improved the stability of encapsulated PSCs,
setting the state of the art for lifetime tests. Such superior device stability is ascribed to the
twofold beneficial role of MoS2, inhibiting both interface and structural aging pathways.
Moreover, I also demonstrated the beneficial role of 2D-MoS2 in the scaling up of this technology,
by realizing large-area cells (>1 cm2). Therefore, my work paves the way towards high efficiency,
large-area and ultra-stable PSCs with lifetimes approaching the industrial standards.
In summary, the research work in my Ph.D. aimed to address the following objectives: (a) Design, synthesis and characterization of 2D materials, e.g., graphene and TMDs
(b) Integration of the as-produced 2D materials in (photo)electrochemical devices and PSCs.
A summary of the principal studies includes: i) Optimization of solution-based exfoliation
processes for the synthesis of 2D materials. ii) Morphological, optical, electrical and electrochemical characterization of the as-produced 2D material dispersions and inks by different techniques, such as optical absorption (OAS), Raman spectroscopy, transmission electron microscopy (TEM) and atomic force microscopy (AFM). iii) Evaluation of 2D materials as
electrocatalysts for hydrogen (H2) evolution reaction (HER). iv) Exploitation of 2D materials as
novel HSL in organic photoelectrochemical cells. v) 2D material-based interface engineering PSCs.
The following dissertation is organized in different chapters, whose content is summarized below.
Chapter 1 provides an overview of 2D materials and their applications. In fact, I underline the
morphological, optical, electrical and electrochemical properties of 2D materials, providing a comparison with their bulk counterparts, and how such properties make them suitable for the design and realization of “next-generation” (opto)electronics and energy devices.
Chapter 2 presents the solution-based exfoliation methods, which I developed and used for the
production of the 2D materials. A particular attention is paid to the LPE process, as it represents the main technique exploited for the production of 2D materials.
25
Chapter 3 reports the physicochemical, electrical and electrochemical characterization of the
as-produced 2D material dispersions.
Chapter 4 shows the use of the as-produced 2D materials as electrocatalysts for HER. In particular,
I investigated the impact of the morphology (lateral size and thickness), crystal structure (material phase), defects and chemical composition (impurity, doping) of 2D materials on the HER-electrocatalytic activity.
Chapter 5 reports the investigation of the use of 2D materials as novel HSL for H2-evolving organic photocathodes, with a deep understanding on their role for increasing both the efficiency and the electrochemical stability of the devices.
Chapter 6 presents 2D material-based interface engineering for increasing the efficiency and the
stability of the PSCs. In particular, it is reported the exploitation of the MoS2 both as HTL (as
replacement of the traditional Spiro-OMeTAD) or as active buffer layer between the perovskite active layer and the Spiro-OMeTAD.
27
CHAPTER 1
CHAPTER 1:Two-Dimensional (2D) Material Families and Their Properties
1.1 Graphene
The building block of all organic materials is carbon, which can form a variety of hybridization
states with the neighboring carbon atoms, such as sp, sp2, sp3. Indeed, due to their valence, the
atoms of carbon can bond together in different ways forming the different carbon allotropes.
Diamond (sp3 hybridization) and graphite (sp2 hybridization) are the best known allotropic forms
because their different physical properties (i.e., hardness, density, electrical and thermal conductivity and transparency). Many more allotropes and forms of carbon have been discovered and investigated in the last decades. Low dimensional carbon allotropes, i.e., nanoallotropes, include fullerenes (0-dimensional (0D)), carbon nanotubes (CNTs) (1-dimensional (1D)) and graphene (2D). Some of the carbon allotropes are presented in Figure 1.1.
Figure 1.1. Representative allotropes of carbon: fullerenes (0D), carbon nanotubes (1D), graphene (2D) and diamond (3D). 1
Typically, the properties of these carbon nanoallotropes (e.g., surface area, electrical and thermal conductivity, mechanical strength, etc.) make them attractive for a wide range of applications. For
example, fullerene represents the state-of-the-art acceptor component for organic photovoltaic,
being also widely exploited as electron transport layer in different types of solar cells, selective
28
investigated materials in the past decade, have been exploited as electrode for batteries,
supercapacitors, fuel cells, biomedical applications and etc.3 Furthermore, the exceptional charge
transport, electrical, optical properties of graphene have opened a new and exciting field of
research and development of carbon-based electronic and optoelectronic devices, chemical
sensors, nanocomposites and energy storage.4
Structurally, graphene consists of a two-dimensional honeycomb network of sp2-hybridized
carbon, with carbon-carbon bond distances of 0.142 nm (Figure 1.2).5 Graphene was firstly studied
theoretically in 1947 by P. R. Wallace, 6 who described the graphene as a zero gap semiconductor
due to its lack of an electronic energy gap and its vanishing density of states at the K point, where the conduction and valence band meet. The qualitative description of the band structure of the
graphene permitted to explain also the conductivity of graphite crystals.6 The research of
graphene grown slowly in the late 20th century hoping to observe superior electrical properties from thin graphite or graphene layers. Various attempts were performed to synthesize graphene including using the same approach for the growth of carbon nanotubes (producing graphite with
hundred layers of graphene).7 However, none of them provided perfect monolayer graphene. It
was until 2004 that Andre Geim and Konstantin Novoselov used a successful method to isolate graphene, by mechanical exfoliation of graphite, i.e., “Scotch-Tape method. They obtained few layers graphene flakes. Following this approach, in 2005, they isolated the first-ever free-standing
graphene flakes only one atom thick. 4,8 This was the starting point for the launch of a new
research field that has brought them to receive the Nobel Prize in Physics in 2010 for
groundbreaking experiments regarding the graphene,9 including the studies of it electronic band
structure.
29
1.1.1 Electronic Properties
The band structure and therefore the electronic properties of graphene can be described by
tight-binding Hamiltonian.11 Because the bonding and anti-bonding σ-bands are well separated in
energy (>10 eV at the Brillouin zone center Γ), they can be neglected in semi-empirical
calculations, retaining only the two remaining π-bands.12 The electronic wave functions from
different atoms on the hexagonal lattice overlap.13 However, the overlap between the pz(π) and
the s or px and py orbitals is strictly zero by symmetry. Consequently, the pz electrons, which form
the π-bonds, can be treated independently from the other valence electrons. With one pz electron
per atom in the π–π* model (the s, px, py electrons fill the low-lying σ-band), the (–) band (Valence
band (VB) π (bonding molecular orbital)) is fully occupied, whereas the (+) (conduction band (CB) π* -antibonding molecular orbital-) branch is totally empty. These occupied and unoccupied bands
touch at the Dirac points (K points), see Figure 1-3a.14 In single layer graphene, the unit cell
consists of two carbon atoms - the A and B sublattices (see Figure 1-3b). The band structure of graphene exhibits two bands intersecting at two inequivalent points K and K′ in the reciprocal space (see Figure 1-3b). Near these points, the electronic dispersion resembles that of relativistic Dirac electrons. For this reason, K and K′ are commonly referred to as the “Dirac
points”. As the valence and conduction bands are degenerate at the Dirac points, graphene is a
zero gap semiconductor.15 Therefore, the Fermi level EF is the zero energy reference, and the
Fermi surface (i.e. an abstract boundary in reciprocal space) is defined by K and K’. The dispersion
relation at K(K’) yields the linear π- and π*-bands for Dirac fermions:11
E
±(κ) = ± ħν
F|κ|
(1)where κ = k – Kand νFis the electronic group velocity, which is given by νF=√3 γ0a/(2ħ) ≈ 106m s–1.
The linear dispersion given by the equation (1) is the solution to the following effective
Hamiltonian at the K(K’) point H = ±ħνF (σ • κ), where κ = –i ∇ and σ are the pseudo-spin Pauli
30
Figure 1.3. a) Schematic diagram of the linear energy band dispersion in graphene at the Dirac points.17 b) Structure of graphene in the real and momentum space.
For a charge carrier moving through an electric field, mobility (µ) is inversely proportional to the
carrier effective mass m*:18
µ =
𝑞
𝑚
∗𝜏
Where q is the charge of the carrier and τ is the average scattering time. Consequently, extremely high values of µ are expected for electrons in graphene, provided their behavior as massless Dirac
fermions, hence free to move for micrometers without scattering at room temperature..19 Such a
low effective mass provides extremely high values of µ, which makes graphene an appealing candidate for many practical applications in electronic devices. As a matter of fact, experimental
results at room temperature have shown mobility values around 15000-20000 cm2 V-1 s-1, 20 hence
much higher compared with silicon, which has values in the order of 1000 cm2 V -1 s-1 .21
1.1.2 Optical Properties
Although graphene is a single atom thick material,22 it can be optically visualized23 and its
transmittance can be expressed in terms of the fine-structure constant.24 The absorption spectrum
of graphene is quite flat from ultraviolet (UV) to infrared (IR), with a peak at ~270 nm, due to the
exciton-shifted van Hove singularity in the density of states.25 This, in principle, allows graphene to
be used over a broad wavelength range (e.g. from UV to THz).26 In few layers graphene (FLG),
other absorption features can be seen at lower energies, compared to single layer graphene,
31
transmittance (T) of a freestanding graphene can be derived by applying Fresnel equations, in the thin film limit, for a material with a fixed universal optical conductance
(G0= e2/4 ħ ≈ 6.08 x 10-5 Ω-1), 12 to give:
T
= (1 + 0.5 𝜋𝛼 )
−2≈ 1 − 𝜋𝛼 ≈ 97.7 %
where 𝛼 = e2/ (4 π ԑ0 ħ c) = G0 / (π ԑ0 c) ≈ 1/137 is the fine-structure constant.24 The absorbance
can be calculated as A = 2 - log10 %T = πα = 2.3 %. Thus graphene reflect < 0.1 % of the incident
light in the visible region25 and it can be considered as a transparent material. Moreover, it is
worth noting that its high T is almost independent from wavelength of the light,28 due to the linear
energy dispersion previously discussed. This is a key advantagecompared to traditional transparent conductors, such as the transparent conductive oxide (TCO) e.g., ITO (Indium Tin Oxide) and fluorine-doped tin oxide (FTO).
1.1.3 Mechanical Properties
The remarkable mechanical properties of graphene are one of the reasons why graphene stands out as an individual material and as a reinforcing agent in composites. Graphene owes these
exceptional mechanical properties to the sp2 bonds that form the hexagonal lattice and opposes a
variety of deformations in the plane29. In 2008, Hone and coworkers30measured, for the first time,
the mechanical properties of free-standing atomically perfect nanoscale monolayer graphene by using nanoindentation in an atomic force microscope (AFM). The authors reported that Young’s modulus of graphene is E= 1.0 ± 0.1 TPa and an intrinsic strength of 130 GPa. Since then, monolayer graphene known as the strongest material ever tested.
Different values of stiffness, probably arising from the inherent crumpling of graphene in the
out-of-plane direction of the monolayer, have been reported3132. Crumpling of graphene is inevitable
emerging from either out-of-plane flexural phonons or from static wrinkling. The latter is caused by the uneven stress at the boundary of the graphene produced, and it is responsible for the deterioration of the mechanical properties of the material. Another possible origin of crumpling of
graphene is the presence of point defects at a finite distance, such as the Stone-Wales defects33.
Crumpling and wrinkling are critical aspects of graphene, and both play a major role in the design of complex nanomechanical systems.
32
Another important mechanical property of graphene is its fracture toughness.29 Zhang et al.
determined the fracture toughness of CVD-synthesized graphene. They proved the fracture stress
decreased with increasing crack length, and the critical strain energy release rate (GC) was found
to be 15.9 J m-2. The fracture toughness of graphene was estimated as a critical stress intensity
factor (KC) of 4.0 ± 0.6 MPa. 34
1.1.4 Thermal Properties
The heat flow direction in a two dimensional graphene can be divided into in-plane and
out-of-plane directions. High in-out-of-plane thermal conductivity is due to covalent sp2 bonding between
carbon atoms, whereas out-of-plane heat flow is limited by weak van der Waals coupling.35
Simulation work was first performed to predict the thermal conductivity of the monolayer
graphene, showed the extremely high value of 6000 W m-1K-1 at room temperature,36 especially if
compared to graphite (2000 W m-1k-1) and diamond (2200 W m-1k-1).35 Many work were later
carried out to obtain the accurate thermal conductivity of the graphene, and it was reported to be
2000-4000 W m-1K-1 35
(when it was found in freely suspended samples). The upper end of this
range is achieved for isotopically purified samples (0.01% 13C instead of 1.1% natural abundance)
with large grains, whereas the lower end corresponds to isotopically mixed samples or those with smaller grain sizes. Naturally, any additional disorder or even residue from sample fabrication will
introduce more phonon scattering and lower these values further.35 It can be seen that graphene
presents an excellent thermal conductivity at room temperature which is highest among the any
known materials such as diamond, graphite, CNT (3000 W m-1K-1 for Multi Walled Carbon
Nanotubes (MWCNT)37 and 3500 W m−1 K−1 in the case of single walled carbon nanotube
(SWCNT).38) or metals (i.e., silver (430 W m-1 k-1) or copper (380 W m-1 k-1)). It is expected that
thermal properties of graphene can be tuned and will be beneficial for thermoelectric applications.
1.2 Other 2D Materials
The amazing properties of graphene, such as excellent electrical and optical properties, 39 sparked
a material revolution around the world. Despite the fascinating properties of graphene, the absence of an electronic bandgap limits its application as active material in logical circuits and in photovoltaics, where semiconducting properties are required. In fact, the very large off-current of graphene at room temperature, due to its zero bandgap, negatively influences the on/off ratio in
33
the transistor. Moreover, although graphene can absorb all the wavelength of the solar spectrum, the effects of thermal relaxation make challenging the use graphene as a light absorber in solar energy conversion devices. On one hand, researchers tried different methods to introduce a
bandgap in graphene, including chemical functionalization40 and nanostructuring,41 However,
these methods either sacrifice the high μ of graphene (for example, 150 meV bandgap causes the
decrease of the μ down to 200 cm2V-1s-1)42 or require very high voltage (100 V opened a 250 meV
bandgap in bilayer graphene)34. In fact, the opening of a band gap in graphene is not
straightforward, mostly affecting the pristine properties of graphene.43 On the other hand, new 2D
materials have been sought-after. In particular, TMDs (e.g., MoS2,WS2, and NbSe2) represent a
large family of layered materials with the formula MX2, where M is a transition metal element
from group IV (Ti, Zr or Hf), group V (V, Nb or Ta) or group VI (Mo, W), and X is a chalcogen atom (S, Se or Te) The monolayer TMDs are particularly interesting due to their direct energy band gaps
and non-centrosymmetric lattice structure.44,45 For example, MoS2 exhibit tunable bandgaps that
can undergo transition from an indirect band gap in bulk crystals to a direct band gap in monolayer
nanosheets (Figure 1-4a).46 Thus, the diverse 2D-TMDs have emerged as an exciting class of
atomically thin semiconductors with tunable electronic structures (Figure 1-4b). The electronic structure of TMDs also exhibits special features except for general characteristics of common semiconductors. Electrons in 2D crystals that have a honeycomb lattice structure possess a pair of
inequivalent valleys in the k-space electronic structure with an extra valley degree of freedom.47 It
is worth noting that a number of TMDs (MX2 ,where M=Mo,W, and X = S, Se) exhibit nearly
identical crystal structures and similar electronic structures, and will provide family of semiconducting atomic membranes for exploring the valley physics. Recent studies on exfoliated flakes of TMDs have shown exciting potential of these atomically thin materials, including the
demonstration of atomically thin transistors,48 vertical tunnelling transistors that may promise
unprecedented switching speed,49 vertical field-effect transistors (VFETs) that could enable 3D
electronic integration,50 as well as new types of optoelectronic devices such as tunable
34
Figure 1.4. a) Energy dispersion in bulk, quadrilayer (4L), bilayer (2L) and monolayer (1L) MoS2 from left to right. The
horizontal dashed line represents the energy of a band maximum at the K point. The red and blue lines represent the conduction and valence band edges, respectively. The lowest energy transition increases with the decreasing layer and evolve from indirect to direct (vertical) transitions.46. b) The relative valence and conduction band edge of some common TMDs (monolayer).48
1.3 Production Techniques of 2D Materials
The successful exploitation of graphene and other 2D materials crucially depends on the
development and optimization of the production methods.53 In general, a large variety of
approaches to produce graphene and 2D materials have been developed so far (see figure 1.5).54
These can be roughly divided in two main typologies of production processes, i.e., bottom-up and
top-down.54
The bottom-up approach consists on the synthesis and growth of graphene/2D materials atom by atom (growth on silicon carbide (SiC), molecular beam epitaxy (MBE), precipitation from metals, chemical synthesis from benzene building block and chemical vapor deposition (CVD)).
The top-down approach for the production of graphene/2D materials consists in the exfoliation of a 3D bulk structure (mechanical cleavage (MC), anodic bonding , photoexfoliation, and liquid phase exfoliation (LPE)).
35
Figure 1.5. Schematic illustration of the main graphene production techniques. (a) Micromechanical cleavage. (b) Anodic bonding. (c) Photoexfoliation. (d) Liquid phase exfoliation.(e) Growth on SiC. Gold and grey spheres represent Si and C atoms, respectively. At elevated T, Si atoms evaporate (arrows), leaving a carbon-rich surface that forms graphene sheets. (f) Segregation/precipitation from carbon containing metal substrate. (g) Chemical vapor deposition. (h) Molecular Beam epitaxy. (i) Chemical synthesis using benzene as building block.54
In the following sections, I will provide further details about the use of SiC, CVD, MC, anodic bonding and LPE in the production of graphene.
1.3.1 Bottom-up Approach:
1.3.1.1 Growth of Graphene on Silicon Carbide (SiC):
The preparation of graphene by the thermal decomposition of SiC has been proposed as one of
the viable routes for the synthesis of uniform and large-scale graphene layers.55 The method of
producing graphite from SiC is known as early as 1896, as reported by Acheson.54 Actually, there is
36
what happens in a traditional epitaxial growth process, in which Si is deposited on the SiC surface, in this growth technique the carbon rearranges itself in a hexagonal structure after the Si
evaporation from the SiC substrate.54 The procedure for the SiC thermal decomposition is
theoretically simply and consists basically of two steps: firstly the samples cleaning is required to
remove surface polishing damage, then the growth starts by thermal treatment of SiC.55For what
concerns the second step, the annealing of the substrates results in the sublimation of the silicon atoms while the carbon-enriched surface undergoes reorganization and, for high enough
temperatures, graphitization.56 The typical range of annealing temperatures goes from 1300°C to
2000°C and the usual heating and cooling rates are 2-3°C/sec.This technique allows to obtain, to
date, graphene domains up to 200 nm in size with mobility at room temperature up to 3·104
cm2V-1s-1. The thermal decomposition, however, is not a self-limiting process and areas of different
film thicknesses may exist on the same SiC crystal but the major short-coming regards the SiC wafers cost that blocks up the breakthrough of this method. A considerable advantage for the technological applications is that SiC, being an insulating substrate, can be simultaneously used as growth and election substrate without transferring the graphitic layers to another insulator substrate, avoiding all the drawbacks due to this process, as will be discussed later, for example,
for the CVD technique.54,55
1.3.1.2 Chemical Vapor Deposition (CVD):
Chemical vapor deposition has been the workhorse for depositing several materials used in semiconductor devices for several decades. It has been widely used to deposit or grow thin films,
crystalline or amorphous, from solid, liquid or gaseous precursors of many substances.57 Chemical
vapor deposition works combining gas molecules in a reaction chamber. While the combined gases come into contact with the substrate, which is heated at elevated temperatures under low pressure, a reaction takes place creating a material film on the substrate surface. The waste gases are then pumped out from the reaction chamber. Since the temperature of the substrate is a primary condition that defines the type of reaction that will occur, it is vital to set it rightly.
The deposition process can include two types of reactions: homogeneous gas phase reactions, which occur in the gas phase, and heterogeneous chemical reactions which occur on/near the vicinity of a heated surface leading to the formation of powders or films, in each case. During the
37
CVD process, the substrate is usually coated a very small amount, at a very slow speed, often described in microns of thickness per hour.
There exist many different types of CVD processes, including thermal, plasma enhanced (PECVD),
cold wall and hot wall.54 The kind of CVD to be used depends on the available precursors, the
material quality, the thickness, and the structure needed.54 The type of precursor is usually
dictated by what is available, what yields the desired film, and what is cost-effective for the specific application. It must be noted that cost is an essential part of selecting a specific process. The principal advantages of using CVD are the high quality and high purity of the films created. This method has been employed as an inexpensive alternative to producing relatively high-quality and large-area graphene. During the CVD process to produce graphene, the gas species (commonly methane, ethylene, or acetylene) are fed into the reaction chamber and pass through the hot zone, where hydrocarbon precursors decompose to carbon radicals at the metal substrate surface and then, form single-layer and few-layer graphene. During the reaction, the metal substrate not only works as a catalyst but also determines the graphene deposition mechanism, which ultimately affects the quality of graphene. Finally, samples are cooled down in argon gas. During the cooling down process, carbon atoms diffuse out from the metal-C solid solution and precipitate on the metal surface to form graphene films.
Another crucial point in the production of graphene by CVD is the selection of the proper metal substrate. In 1966, Karu and Beer demonstrated that Ni substrates, exposed to methane at T = 900
°C, can be used to form thin graphite58.Later, in 1984, Kholin et al. performed what may be the
first CVD graphene growth on a metal surface59. The authors selected iridium (Ir) as the metal
substrate to study the catalytic and thermionic properties of the metal in the presence of
carbon60. Since then, other groups exposed metals, such as single crystal Ir61,62, to carbon
precursors and studied the formation of graphitic films in ultra-high vacuum (UHV) systems. It was after 2004 when the focus of the scientific community shifted to the actual growth of graphene. It was found that low-pressure chemical vapor deposition (LPCVD) on Ir(III) single crystals, using an ethylene precursor, yields graphene structurally coherent even over the Ir step
edges61. However, the transfer of graphene to other substrates is a complicated process,
influenced mainly because of the chemical inertness of this metal. Additionally, Ir is also very
expensive. For this reason, the growth of graphene by CVD by using less costly metals such as Ni63
38
poses a different challenge since few layer graphene are usually grown, and single layer graphene grow non-uniformly. The process is, in fact, carbon precipitation, not yielding uniform single layer
graphene, but rather few layer graphene, and not CVD growth as many papers claim 54,60,62.
The first CVD growth of uniform, large area (∼cm2) graphene on a metal surface was in 200966.
The authors grew graphene on polycrystalline Cu foils, exploiting the thermal catalytic decomposition of methane and low carbon solubility. During the CVD process, the growth mostly
ends as soon as the Cu surface is entirely covered with graphene67. Large area graphene growth
was enabled principally by the low C solubility in Cu68, and the Cu mild catalytic activity69. Indeed,
the solubility of carbon in transition metal along with CVD conditions plays an important role in determining growth mechanism and ultimately controls the number of graphene layers.
The growth mechanism depends on the nature of the catalyst.70 The difference in the growth
kinetics and mechanism between metal substrates was first ascribed to the different carbon solubility. However, the mechanism is more complicated. Carbon atoms, after decomposition from
hydrocarbons, nucleate on Cu, and the nuclei grow into large domains.71,72 The nuclei density
depends on T and pressure. In fact, at low precursor pressure, mTorr, and T > 1000 °C, very large
single crystal domains, ∼0.5 mm, are observed.70 However, once the Cu surface is fully covered,
the films become polycrystalline. This can be associated with the fact that the nuclei are not
registered.70 For example, the cores are incommensurate to each other, even on the same Cu
grain. The latter could be attributed to the low Cu-C binding energy73. It would be desirable to
have substrates (e.g., Ru) with higher binding energy with C.72 However, while Ru is compatible
with Si processing74, oriented Ru films may be challenging to grow on large diameter (300 – 450
mm) Si wafers, or transferred from other substrates.
The surface roughness of the metal substrate is another issue to take into consideration. Commercial Cu foils have been used for the graphene synthesis to reduce the overall cost of fabrication process but these foils have strongly corrugated surface due to cold rolling process during manufacture. The surface roughness is known to produce graphene thickness variation on
Cu75. Since graphene growth on copper is surface-limited, the smoothness of the chosen metal
surface is critical for obtaining monolayer coverage across the entire surface of the
substrate.76 Another major problem with CVD is that graphene is obtained on top of a metal
39
application (i.e., insulating substrates). Moreover, inevitable structural damage occurs to graphene
during the transfer process, which can thus degrade its properties.77
1.3.2 Top-Down Approach:
1.3.2.1 Mechanical Cleavage (MC):
The mechanical cleavage or exfoliation can be regarded as the mother of all techniques for the graphene production, since it was the way that allowed Geim and co-workers at Manchester
University in 2004, to isolate the first single-layer samples from graphite.78
It consists basically in the exfoliation of a graphite block, highly oriented pyrolytic graphite (HOPG) or other types, through the adhesive tape so that the method has been universally known as “the scotch-tape technique”. Then, the first piece of tape is repeatedly cleaved by other sticky pieces down to obtain an almost invisible powder on the starting tape. The number of the exfoliations
ranges from 10 to 20 but,79 a trade-off between this number, namely the flake thickness, and the
mean size needs to be reached. Finally, at the end of the exfoliation process, the tape is
transferred onto the election substrate that usually is silicon dioxide on Si (SiO2/Si).14
Ideally, the single-layer graphene can be obtained making thinner and thinner the thickness of the
graphite block. However, transferring the adhesive tape to the SiO2/Si implicates that also glue
residues can be released on the substrate. Besides eliminating the glue residues, was able to
increase the mean flake size from ten up to hundreds of microns.80 Although MC is impractical for
large scale applications, it is still the method of choice for fundamental studies. Indeed, the vast majority of basic results and prototype devices were obtained using MC flakes. Thus, MC remains
ideal to investigate both new physics and new device concepts.54
1.3.2.2 Anodic bonding
Anodic bonding is widely used in the microelectronics industry to bond Si wafers to glass, to protect them from humidity or contaminations. When employing this technique to produce single layer graphene, graphite is first pressed onto a glass substrate, and a high voltage of few KVs (0.5-2 kV) is applied between the graphite and a metal back contact, and the glass substrate is then
heated (∼200 ◦C for∼ 10-20 mins). If a positive voltage is applied to the top contact, a negative
40
Na2O impurities in the glass into Na+ and O−2 Ions. Na+ moves towards the back contact, while O−2
remains at the graphite-glass interface, establishing a high electric field at the interface. A few layers of graphite, including single layer graphene, stick to the glass by electrostatic interaction
and can then be cleaved off; temperature and applied voltage can be used tocontrol the number
of layers and their size. Anodic bonding has been reported to produce flakes up to about a
millimeter in width.54
1.3.2.3 Liquid Phase Exfoliation (LPE)
Liquid-phase exfoliation of graphite is based on exposing powdered graphite to special solvents or surfactants that favor an increase in the total area of graphite crystallites. Solvents ideal to disperse graphene are those that minimize the interfacial tension [mN/m] between the liquid and
graphene flakes, i.e. the force that minimizes the area of the surfaces in contact.54 The solvents
that mainly match this requirement are N-methyl-pyrrolidone (NMP), Dimethylformamide
(DMF).81
The second step of the procedure consists in the ultra-sonication aimed to favor the splitting of graphite into individual platelets. Finally, a “purification” step is required to separate the unexfoliated flakes from the thinner ones, constituting the so called surnatant phase of the suspension. Thicker flakes can be removed by different strategies based on ultra-centrifugation in
a uniform medium or in a density gradient medium.54 (see figure 1.6)
In this context, in the first step, the choice of the solvent for the exfoliation process is crucial. In fact, suitable solvents are those that minimize the interfacial tension between the liquid and the flakes in solution. In general, interfacial tension plays a key role when a solid surface is immersed in a liquid medium. If the interfacial tension between solid and liquid is high, there is poor dispersibility of the solid in the liquid. In the case of graphitic flakes in solution, if the interfacial tension is high, the flakes tend to adhere to each other and the work of cohesion between them is high (i.e. the energy per unit area required to separate two flat surfaces from contact), hindering
their dispersion in liquid.54 For example, graphene flakes have surface energy (γ) of ~ 40 mN m-1,
thus suitable solvents are NMP and DMF. However, these solvents are toxic and have high boiling point, i.e., more than 150 °C. A possible solution to this issue relies on the tuning of γ parameter of lower boiling point solvents, such as acetone and ethanol, by adding stabilizing agents, such as surfactants or polymers. However, their residual can increase the inter-flake contact resistance.
